Method for casting polymeric gels from volatile mixtures of monomer in open molds and apparatus for performing this method

ABSTRACT

The invention pertains to a method for casting polymeric gels from volatile mixtures of monomers in open molds and to apparatus for performing this method. 
     The principle of the invention resides in the fact that the same concentrations of volatile components present in the polymerization mixture are also present in the gaseous state within a protective gas maintained above the free surface of the polymerization mixture, said gaseous volatile components being in the equilibrium state with those in the polymerization mixture at polymerization temperature. 
     The apparatus for performing said method comprises a main pipe branch which introduces a pure protective gas into a polymerization apparatus and at least one auxiliary pipe branch for saturating the protective gas with a volatile component. These main and auxiliary pipe branches are formed by an inlet pipe connected to a control valve for setting a constant volume flow-rate and further leading to a device for measurement of the flow rate, where their outlet pipes are connected to a common inlet entering the polymerization apparatus and a saturator for a pure volatile component of the monomer mixture is arranged within the auxiliary branch before the outlet pipe.

BACKGROUND OF THE INVENTION

The invention pertains to a method for casting polymeric gels fromvolatile mixtures of monomers in open molds and to an apparatus forperforming this method.

One of the most efficient methods of manufacturing shaped articles fromplastics is the casting of monomers or their mixtures in molds underpolymerization conditions. The method of casting in open molds can beused for some purposes in particular advantageously, for example, forcasting foils on horizontal plates or for casting contact lenses inrotating open molds demarcated by a sharp edge. The latter method hasbeen so far applicable only for such starting monomer mixtures wherenone of the components are pronouncedly volatile under the conditions ofthe entire procedure. However, if even only one of the monomers used, ora single component of the solution of monomer mixture employed, isvolatile under the polymerization conditions, the more volatilecomponents will evaporate from the open surface of the cast mixture andthe composition of the surface layers will change. A casting obtained inthis way is a substantially nonhomogeneous composition with the resultthat the casting is extensively deformed.

Thus, for example, contact lenses prepared by centrifugal casting of amonomer mixture of hydroxyethyl methacrylate and acrylic acid in thepresence of a crosslinking agent or in the presence of inert solventsalways perform more poorly due to the lower hydrophilicity of theirinner surface caused by loss of the more hydrophilic component (i.e.,acrylic acid), and such cast lenses always roll up in an uncontrollableway. Similarly, gels cast from a mixture of N-vinyl-pyrrolidone andmethyl methacrylate lose the much more volatile methyl methacrylatecomponent at the free surface. As a consequence, the upper layers aremore swellable with water and the lenses are sometimes deformed to suchan extent that they turn inside out in an uncontrollable way afterswelling.

Similar difficulties also occur in the polymerization of substantiallynonvolatile monomers which are diluted with volatile solvents. Forexample, if water is used as a solvent in the polymerization of mixturesof hydroxyethyl methacrylate with sodium methacrylate, the mixturebecomes concentrated at its free surface by evaporation of water so thatthe resulting hydrophilic gel, brought into equilibrium with water, ismore contracted on the originally open side than on the other sidethereby causing its extensive deformation.

These difficulties also occur to a lesser extent when the polymerizationof the monomer mixture is carried out with an open surface but in closedmolds if this surface comes into contact with a protective gas beforethe mold is closed. It is possible for the protective gas to remove avolatile component from the surface of the monomer mixture over ashorter or longer period of time and at the very least, the mixture mustbe saturated to equilibrium with this component in the closed mold andits concentration at the surface reduced in this way.

SUMMARY OF THE INVENTION

The above difficulties are completely or almost completely overcome bythe method according to the invention, which consists in adding anamount of each volatile component of the monomer mixture to a protectivegas which is provided above the free surface of the monomer mixture inthe molds, said amount of volatile component being sufficient to attainthe partial pressure of each volatile component which corresponds to thepartial pressure of said component above the surface of the monomermixture.

The protective gas is mixed, in the method according to the invention,in a predetermined ratio with one or more streams of a protective gaswhich is saturated by being passed through a pure volatile componentcontrolled to an accurate temperature which is advantageously theworking temperature of the apparatus. Nitrogen, argon, carbon dioxide orother inert gas, or their mixtures, are advantageously used as theprotective gas.

BRIEF DESCRIPTION OF THE DRAWING

The invention is further illustrated in the attached drawing which showsdiagrammatically one embodiment of apparatus useful in carrying out themethod of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus for performing the foregoing method consists of a mainpipe branch for introducing the pure protective gas into apolymerization apparatus and at least one auxiliary pipe branch forsaturating the protective gas with the volatile component. Thesebranches are formed by an inlet pipe connected with a control valve forsetting a constant volume rate of flow and a device for flow measurementplaced behind this valve and the pipe outlets are combined into a commoninlet into the polymerization apparatus. A saturator for the purevolatile component of the monomer mixture is placed before the outletpipe of the auxiliary branch. The inlet pipe of the auxiliary branch isadvantageously connected to the inlet pipe of the main branch. Thesaturator for the pure volatile component of the monomer mixture may beplaced behind the device for flow measurement. A sprinkler column with acirculation pump or a labyrinth saturator with a free level (to minimizedroplet entrainment) may be used, for example, as a saturator. Thevolume flow rate can be measured, e.g., by rotameters or Venturi tubes.The number of auxiliary branches with saturators corresponds to thenumber of volatile components (monomers or solvents) present in themonomer mixture.

The vapor pressure of individual components above the given monomermixture can be relatively easily measured by the analysis of carrier gasbrought into equilibrium with the mixture. In addition to this,sufficiently accurate and relatively reliable information can also beobtained by calculations, e.g., by the method of group contributionsknown as "UNIFAC" (Fredenslund A., Gmehling J., Rasmussen P.:Vapor-Liquid Equilibria Using UNIFAC, Elsevier, Amsterdam, 1977). Thismethod assumes that the liquid phase is formed by a solution ofmolecular groups, the contribution of which to the total activitycoefficient of the component can be quantified. Numerical data on theextent of the group interactions required for this calculation werealready twice revised and expanded (Gmehling J., Rasmussen P.Fredenslund A.: Ind. Eng. Chem., Process Des. Develop. 21, 118-127(1982)). Reliability of this method may be considerably enhanced if itis applied to solutions with a limited system of groups (similar typesof solutions), where a subsystem of contributions is calculated fromactivity coefficients of systems, which contain just the needed groups.

Relationship for the ideal solution may be used as a completelysufficient approximation under the conditions of calculation whichshould be carried out here. To calculate the partial pressure P of thecomponent above the solution, only the knowledge of the pressure of thesaturated vapor P° of a pure component is required at the giventemperature and the amount of component in the liquid phase as a molefraction x. Partial pressure of the component is then P=xP°.

Modification of the protective gas, i.e., its enrichment with vapors ofvolatile component, may also be carried out in other ways. The simplest,but a relatively costly method, consists in bubbling a finely dispersedgas through a large amount of the given monomer mixture which isstabilized against polymerization by addition of a nonvolatileinhibitor, e.g., copper (I) chloride, in one or more saturators. But thepreviously described method of this invention is much more advantageousand economical in operation.

The protective gas can be modified according to the invention by thisespecially advantageous method in two particular ways:

(1) Lowering the degree of saturation by decreasing the temperature ofthe saturator, i.e., the lowered temperature (t₂) of the saturator canbe calculated from the temperature of the apparatus environment (t₁) ifthe temperature dependence of the pressure of saturated vapor is known,by means of the relationship

    p°(t.sub.2)=xp°(t.sub.1).

(2) Lowering of the degree of saturation by gas dilution. The protectivegas is saturated in a saturator at a suitable constant temperature andthe degree of saturation is lowered by dilution with the pure protectivegas in the following ratio (of volumes, flow rates, and the like):

    pure protective gas/saturated protective gas=(1/x)-1.

The protective gas may be saturated by the above described methods witha pure volatile component the concentration of which in the mixture isgiven as a mole fraction x. If the liquid contains more volatilecomponents, the protective gas is formed by mixing the gas streamssuitably saturated with individual components.

Thus, for example, 500 ml/min of a protective gas for the monomermixture containing 2 mol-% acrylic acid as the volatile component can beprepared as follows: Rinsing of the apparatus and polymerization may becarried out at 35° C., when pure acrylic acid has the pressure ofsaturated vapor 1.036 kPa, i.e., its partial pressure above the givenmonomer mixture is (2/100).1.036-0.0207 kPa. If the saturator withacrylic acid has also the controlled temperature 35° C., then the flowrates of the pure protective gas and the protective gas saturated withacrylic acid have to be in the ratio (1/0.02)-1=49:1. This means thatfor the total flow rate 500 ml/min, the flow rates in individualbranches must be: 490 ml/min pure protective gas (the main pipe branch)and 10 ml/min saturated protective gas (the auxiliary pipe branch).

The apparatus diagrammatically shown in the drawing consists of a mainpipe branch leading the pure protective gas from a source thereof intopolymerization apparatus 1 and of an auxiliary pipe branch forsaturation of the protective gas with a volatile component.

The main pipe branch is formed by an inlet pipe 3 connected to a controlvalve 4 which serves for setting a constant volume flow rate of gas andfurther leading to a device 5 for measuring the flow rate of gas whichis connected to an outlet pipe 6.

The auxiliary pipe branch is formed by an inlet pipe 7 connected to acontrol valve 8 which serves for setting a constant volume flow rate andleads into a device 9 for measuring the flow rate which is connected toa temperature-controlled saturator 10 for a pure volatile component ofthe monomer mixture. Saturator 10 is coupled to outlet pipe 6 of pureprotective gas through an outlet pipe 11 so that a common inlet 12 topolymerization apparatus 2 results. Inlet pipe 7 of the auxiliary branchmay be connected with inlet pipe 3 of the main branch, as is shown inthe drawing, or it may be connected to an independent source ofprotective gas. The saturator can be connected to the auxiliary brancheither before or behind device 9 for measurement of the flow rate. Ifthe monomer mixture contains several volatile components, furtherauxiliary branches with control valves, devices for flow-ratemeasurement and saturators are connected in parallel between the source1 of protective gas and the common inlet 12. In this case, as manysaturators are inserted as there are volatile components in the monomermixture. These auxiliary branches need not necessarily be connected tosingle common source 1 of protective gas but may have independentsources of protective gas.

In operation, the pure protective gas is led from source 1 thereofthrough inlet pipe 3 of the main branch and through inlet pipe 7 of theauxiliary branch via control valves 4 and 8 which are used for settingthe required through-flow of protective gas (determined, e.g., by theabove described calculation) checked with devices 5 and 9 for themeasurement of through-flow of gas. The gas then passes in part directlyinto outlet pipe 6 of pure protective gas, partly through saturator 10,where it is saturated with the volatile component, into outlet pipe 11.The flows from the individual branches are combined and led throughcommon inlet 12, where the pure protective gas and the gas saturatedwith the volatile component are mixed and homogenized, intopolymerization apparatus 2.

We claim:
 1. A method of casting a polymeric gel in an open mold from aliquid polymerization mixture containing at least one volatile monomercomponent dissolved therein which comprises providing a protective gasabove the open surface of the polymerization mixture duringpolymerization, said protective gas prior to coming into contact withsaid open surface of the polymerization mixture being combined with anamount of said volatile monomer which wil be in equilibrium with thevolatile monomer present within the polymerization mixture underpolymerization conditions.
 2. The method of claim 1 whereinpolymerization is carried out in a rotating mold.
 3. The method of claim1 wherein the protective gas is at least one inert gas selected from thegroup consisting of nitrogen, argon and carbon dioxide.
 4. The method ofclaim 1 wherein the polymerization mixture contains hydroxyethylmethacrylate, acrylic acid and a crosslinking agent, the volatilemonomer being acrylic acid.
 5. The method of claim 1 wherein theprotective gas is at least one inert gas selected from the groupconsisting of nitrogen, argon and carbon dioxide.
 6. The method of claim3 wherein the polymerization mixture contains hydroxyethyl methacrylate,acrylic acid and a crosslinking agent, the volatile monomer beingacrylic acid.
 7. The method of claim 1 wherein polymerization is carriedout in a rotating mold, the protective gas is selected from the groupconsisting of nitrogen, argon and carbon dioxide and the polymerizationmixture contains hydroxyethyl methacrylate, acrylic acid and acrosslinking agent, the volatile monomer being acrylic acid.
 8. Themethod of claim 1 wherein the polymerization mixture contains N-vinylpyrrolidone and methyl methacrylate, the volatile monomer being methylmethacrylate.
 9. The method of claim 2 wherein the polymerizationmixture contains N-vinyl pyrrolidone and methyl methacrylate, thevolatile monomer being methyl methacrylate.
 10. The method of claim 3wherein the polymerization mixture contains N-vinyl pyrrolidone andmethyl methacrylate, the volatile monomer being methyl methacrylate. 11.The method of claim 1 wherein polymerization is carried out in arotating mold, the protective gas is at least one gas selected from thegroup consisting of nitrogen, argon and carbon dioxide and thepolymerization mixture contains N-vinyl pyrrolidone and methylmethacrylate, the volatile monomer being methyl methacrylate.
 12. Themethod of claim 1 wherein the polymeric gel is cast in the form of acontact lens.
 13. The method of claim 12 wherein polymerization iscarried out in a rotating mold.
 14. The method of claim 13 wherein thepolymerization mixture contains hydroxyethyl methacrylate, acrylic acidand a crosslinking agent, the volatile monomer being acrylic acid. 15.The method of claim 13 wherein the polymerization mixture containsN-vinyl pyrrolidone and methyl methacrylate, the volatile monomer beingmethyl methacrylate.